The present invention relates to a method and an apparatus for establishing a calibrated reference value representing a threshold and for comparing the reference value with a signal representing a variable parameter to determine the variable parameter's condition.
A wide variety of methods exist for calibrating a reference value which can be used to determine when a signal representing a variable parameter increases past a predetermined threshold. In a typical method using an electronic system, a transducer of a sensor circuit detects a variable physical parameter and produces a corresponding sensor voltage signal (or current signal) that represents the condition of the physical parameter. The sensor voltage signal is then compared with a reference voltage representing the reference value. Typically, the reference voltage is provided by an electronic calibration circuit that includes a precision potentiometer having an adjustable wiper that divides a supply voltage. The reference voltage is established by directing the physical parameter to the desired level and adjusting the potentiometer until the output voltage from the potentiometer equals the sensor voltage signal when the physical parameter is at the desired level. Typically, a comparator circuit is used to compare the reference voltage with the sensor voltage signal.
The method for setting a reference voltage described briefly above requires directing the physical parameter to assume the desired threshold. Sometimes, however, this may not be possible. For example, an accurate determination of the physical parameter may not be available or may be available only for two extreme conditions of the physical parameter. Alternatively, setting and holding the physical parameter at the threshold may be burdensome, inefficient, or otherwise uneconomical. When the physical parameter cannot be held at the threshold, then the reference voltage must be otherwise determined in order for the circuit to be able to indicate when the physical parameter has achieved the calibrated threshold.
Therefore, there is a need to develop a simple method and apparatus to accurately establish a calibrated reference voltage representing a threshold without requiring the physical parameter to be at the corresponding threshold before the reference can be calibrated. In addition, there is a need to develop a method and apparatus to monitor the physical parameter and indicate whether the threshold has been overcome.